This invention relates generally to instruments for converting electrical current levels to fluid pressure.
In process control systems, it is often necessary to convert an electrical signal into a pressure signal. One well-known application of this technology is in a current to pressure transducer used, for example, in actuating an adjustable mechanical valve for flow control. A common form of electrical signal to pressure converter or transducer is the DC current amplitude (I) to pneumatic pressure (P) converter known in the process instrumentation art as an I/P motor, often coupled with a pressure booster, feedback control and positioner, in which case the combination is referred to as an electropneumatic positioner. In known I/P motors for such positioners, an armature of magnetizable material is suspended in a magnetic circuit closely spaced from a nozzle through which a pressurized pneumatic line is controllably vented. The armature and nozzle together form a flapper type pneumatic valve. The magnetic circuit includes an electromagnetic coil assembly which produces a variable magnetic field corresponding to the amplitude of an electrical current (i.e., the electrical input signal) applied to the coil. With a given current input level, the gap between armature and nozzle remains relatively constant, as the combination of the net magnetic force acting on the armature together with any resilience in its suspension just balances the force of the nozzle blast on the armature. However, in response to a change in the magnetic field experienced by the armature, the armature will move either closer to or farther away from the nozzle, blocking the nozzle blast either more or less, thus increasing or decreasing the pressure in the line as a function of the DC electrical current level.